A DMZ (Demilitarized Zone) is a combination of firewalls -- a perimeter network segment logically between internal and external networks. Also called a "screened subnet," its purpose is to enforce the internal network's IA policy for external information exchange and to provide external, untrusted sources with restricted access to releasable information while shielding internal networks from outside attacks. . . .
n the limited space available here, I cannot possibly address how to secure a firewall. Instead, I'll note the considerations that go into doing so and point you to some useful resources. CNSS Instruction No. 4009, revised May 2003, National Information Assurance (IA) Glossary defines a firewall as a "system designed to defend against unauthorized access to or from a private network." I prefer CERT's definition: "A combination of hardware and software used to implement a security policy governing the network traffic between two or more networks, some of which may be under your administrative control (e.g., your organization's networks) and some of which may be out of your control (e.g., the Internet)."

A DMZ (Demilitarized Zone) is a combination of firewalls -- a perimeter network segment logically between internal and external networks. Also called a "screened subnet," its purpose is to enforce the internal network's IA policy for external information exchange and to provide external, untrusted sources with restricted access to releasable information while shielding internal networks from outside attacks. In some circles the DMZ is considered a part of the firewall, while other circles consider the DMZ the land of the sacrificial hosts. One way to think of a DMZ is as a group of hosts that are guided by a unique security policy. This policy balances some of the strictest controls against public access and availability requirements.